With an annual market growth rate of 6.6% over decades, separation films are manufactured in various forms and find applications in a variety of fields including seawater desalination, power generation by salinity gradient, redox cells, fuel cells, etc. Now, separation films are divided into polymer, ceramic, and metal separation films according to the material type thereof, and two or more different types thereof may be used in combination according to purposes. The function of a separation film depends on the pore size thereof. That is, particles can permeate through a separation film if their size is smaller than a pore size of the separation film whereas particles larger than the pore size cannot permeate. In addition, since there is very close relationship between porosity and selective permeability, porosity is also regarded as a very important factor for selective separation.
According to pore size, separation films are classified into reverse osmosis, nanofiltration, microfiltration, and ultrafiltration films. Such separation films are manufactured into various forms according to material properties thereof.
At present, there are five known methods for manufacturing separation films. First, a sintering process is used in which material powder is placed in a module, heated to a temperature slightly lower than the melting point, and sintered under a pressure to give a microfiltration film with a thickness of 100˜500 μm. However, the film manufactured in this process has a porosity of as low as 10˜40%, and is heterogeneous in pore morphology with a broad pore size distribution.
Second, a drawing process is used for manufacturing separation films. In the drawing process, a flat-sheet membrane or a hollow fiber membrane made of a crystalline material (particular polymeric material) is drawn to provide porosity. According to this process, a non-crystalline portion is oriented in the drawing direction to form fine fibrils. In this process, the porosity of the separation film can be increased to up to 90% and the pore size can be controlled according to an extent of drawing. However, materials applicable to the drawing process are limited, and the separation film becomes non-uniform in pore size depending on the extent of drawing.
Third, a separation film can be manufactured using a track etching technique in which a high-energy beam is irradiated onto a polymer film. This technique can establish the most uniform pores, but is complex and limits a film thickness available for the radiation energy. In addition, the track etching technique cannot be applied to various separation films, as understood from the fact that thus far the technique has been applied only to polycarbonate and polyester films.
Fourth, a solvent exchange method is most frequently adopted for the preparation of hollow fibers. This method, which is a phase inversion membrane preparation method, takes advantage of the concept that polymers can precipitate by solvent/non-solvent exchange. This method enables porous hollow fiber membranes to be manufactured in a single process. In the solvent exchange method, phase separation and phase change can be uniformly controlled to some extent, but the membranes exhibit a relatively broad spectrum of pore size distributions. Although now popularly used for seawater desalination, hollow fiber membranes are found to have an ion permeability of 95% or less due to the broad pore size distribution. Further, the method, based on solvent phase separation, is limited for available solvents, which, in turn, makes it difficult to prepare separation films from various materials.
Finally, a thermally induced phase inversion process was developed to expand available materials. Because it utilizes heat rather than conventional phase inversion techniques in forming pores, the thermally induced inversion process can artificially control pore sizes. However, this process is also limited in forming uniform pore sizes.
The performance of a separation film entirely depends on its pore size, pore size distribution, and porosity. Capable as it is of achieving a narrow pore size distribution, a track etching technique is difficult to apply to mass production. The other techniques, although allegedly reported to allow for the formation of uniform pore sizes, are observed to form pores with a wide pore size distribution. Substantially, the films manufactured by the aforementioned techniques exhibit a selectivity of 95% or less. For use as drinking water, for example, the water must be perfectly free of harmful matter, but the term “a selectivity of 95%” means that the film cannot completely remove harmful matter. Thus, the formation of uniform pore sizes in separation films, although recognized by all manufacturers, is a great problem that has yet to be solved.